Three layer NOx adsorber catalyst

ABSTRACT

A lean NOx trap catalyst and its use in an emission treatment system for internal combustion engines is disclosed. The lean NOx trap catalyst comprises a first layer, a second layer, and a third layer.

FIELD OF THE INVENTION

The invention relates to a lean NO_(x) trap catalyst, a method oftreating an exhaust gas from an internal combustion engine, and emissionsystems for internal combustion engines comprising the lean NO_(x) trapcatalyst.

BACKGROUND OF THE INVENTION

Internal combustion engines produce exhaust gases containing a varietyof pollutants, including nitrogen oxides (“NO_(x)”), carbon monoxide,and uncombusted hydrocarbons, which are the subject of governmentallegislation. Increasingly stringent national and regional legislationhas lowered the amount of pollutants that can be emitted from suchdiesel or gasoline engines. Emission control systems are widely utilizedto reduce the amount of these pollutants emitted to atmosphere, andtypically achieve very high efficiencies once they reach their operatingtemperature (typically, 200° C. and higher). However, these systems arerelatively inefficient below their operating temperature (the “coldstart” period).

One exhaust gas treatment component utilized to clean exhaust gas is theNO_(x) adsorber catalyst (or “NO_(x) trap”). NO_(x) adsorber catalystsare devices that adsorb NO_(x) under lean exhaust conditions, releasethe adsorbed NO_(x) under rich conditions, and reduce the releasedNO_(x) to form N₂. A NO_(x) adsorber catalyst typically includes aNO_(x) adsorbent for the storage of NO_(x) and an oxidation/reductioncatalyst.

The NO_(x) adsorbent component is typically an alkaline earth metal, analkali metal, a rare earth metal, or combinations thereof. These metalsare typically found in the form of oxides. The oxidation/reductioncatalyst is typically one or more noble metals, preferably platinum,palladium, and/or rhodium. Typically, platinum is included to performthe oxidation function and rhodium is included to perform the reductionfunction. The oxidation/reduction catalyst and the NO_(x) adsorbent aretypically loaded on a support material such as an inorganic oxide foruse in the exhaust system.

The NO_(x) adsorber catalyst performs three functions. First, nitricoxide reacts with oxygen to produce NO₂ in the presence of the oxidationcatalyst. Second, the NO₂ is adsorbed by the NO_(x) adsorbent in theform of an inorganic nitrate (for example, BaO or BaCO₃ is converted toBa(NO₃)₂ on the NO_(x) adsorbent). Lastly, when the engine runs underrich conditions, the stored inorganic nitrates decompose to form NO orNO₂ which are then reduced to form N₂ by reaction with carbon monoxide,hydrogen and/or hydrocarbons (or via NH_(x) or NCO intermediates) in thepresence of the reduction catalyst. Typically, the nitrogen oxides areconverted to nitrogen, carbon dioxide and water in the presence of heat,carbon monoxide and hydrocarbons in the exhaust stream.

PCT Intl. Appl. WO 2004/076829 discloses an exhaust-gas purificationsystem which includes a NO_(x) storage catalyst arranged upstream of anSCR catalyst. The NO_(x) storage catalyst includes at least one alkali,alkaline earth, or rare earth metal which is coated or activated with atleast one platinum group metal (Pt, Pd, Rh, or Ir). A particularlypreferred NO_(x) storage catalyst is taught to include cerium oxidecoated with platinum and additionally platinum as an oxidizing catalyston a support based on aluminium oxide. EP 1027919 discloses a NO_(x)adsorbent material that comprises a porous support material, such asalumina, zeolite, zirconia, titania, and/or lanthana, and at least 0.1wt % precious metal (Pt, Pd, and/or Rh). Platinum carried on alumina isexemplified.

In addition, U.S. Pat. Nos. 5,656,244 and 5,800,793 describe systemscombining a NO_(x) storage/release catalyst with a three way catalyst.The NO_(x) adsorbent is taught to comprise oxides of chromium, copper,nickel, manganese, molybdenum, or cobalt, in addition to other metals,which are supported on alumina, mullite, cordierite, or silicon carbide.

PCT Intl. Appl. WO 2009/158453 describes a lean NO_(x) trap catalystcomprising at least one layer containing NO_(x) trapping components,such as alkaline earth elements, and another layer containing ceria andsubstantially free of alkaline earth elements. This configuration isintended to improve the low temperature, e.g. less than about 250° C.,performance of the LNT.

US 2015/0336085 describes a nitrogen oxide storage catalyst composed ofat least two catalytically active coatings on a support body. The lowercoating contains cerium oxide and platinum and/or palladium. The uppercoating, which is disposed above the lower coating, contains an alkalineearth metal compound, a mixed oxide, and platinum and palladium. Thenitrogen oxide storage catalyst is said to be particularly suitable forthe conversion of NO_(x) in exhaust gases from a lean burn engine, e.g.a diesel engine, at temperatures of between 200 and 500° C.

Conventional lean NO_(x) trap catalysts often have significantlydifferent activity levels between activated and deactivated states. Thiscan lead to inconsistent performance of the catalyst, both over thelifetime of the catalyst and in response to short term changes inexhaust gas composition. This presents challenges for enginecalibration, and can cause poorer emissions profiles as a result of thechanging performance of the catalyst.

As with any automotive system and process, it is desirable to attainstill further improvements in exhaust gas treatment systems. We havediscovered a new NO_(x) adsorber catalyst composition with improvedNO_(x) storage and conversion characteristics, as well as improved COconversion. It has surprisingly been found that these improved catalystcharacteristics are observed in both the active and deactivated states.

SUMMARY OF THE INVENTION

In a first aspect of the invention there is provided a lean NO_(x) trapcatalyst, comprising:

-   -   i) a first layer, said first layer comprising one or more        platinum group metals, a first ceria-containing material, an        alkali or alkaline earth metal, and a first inorganic oxide;    -   ii) a second layer, said second layer comprising one or more        noble metals, a second ceria-containing material, and a second        inorganic oxide; and    -   iii) a third layer, said third layer comprising one or more        noble metals having reducing activity and a third inorganic        oxide.

In a second aspect of the invention there is provided an emissiontreatment system for treating a flow of a combustion exhaust gascomprising the lean NO_(x) trap catalyst as hereinbefore defined.

In a third aspect of the invention there is provided a method oftreating an exhaust gas from an internal combustion engine comprisingcontacting the exhaust gas with the lean NO_(x) trap catalyst ashereinbefore defined.

Definitions

The term “washcoat” is well known in the art and refers to an adherentcoating that is applied to a substrate, usually during production of acatalyst.

The acronym “PGM” as used herein refers to “platinum group metal”. Theterm “platinum group metal” generally refers to a metal selected fromthe group consisting of ruthenium, rhodium, palladium, osmium, iridiumand platinum, preferably a metal selected from the group consisting ofruthenium, rhodium, palladium, iridium and platinum. In general, theterm “PGM” preferably refers to a metal selected from the groupconsisting of rhodium, platinum and palladium.

The term “noble metal” as used herein refers to generally refers to ametal selected from the group consisting of ruthenium, rhodium,palladium, silver, osmium, iridium, platinum, and gold. In general, theterm “noble metal” preferably refers to a metal selected from the groupconsisting of rhodium, platinum, palladium and gold.

The term “noble metal having reducing activity” as used herein refers toa metal selected from the group consisting of ruthenium, rhodium,palladium, silver, osmium, iridium, platinum, and gold that is known tobe catalytically active in reduction reactions, e.g. the reduction ofNO_(x) gases to NH₃ or N₂, or of CO₂ or CO to hydrocarbons. In general,the term “noble metal having reducing activity” preferably refers to ametal selected from the group consisting of ruthenium, rhodium, iridium,platinum and palladium.

The term “mixed oxide” as used herein generally refers to a mixture ofoxides in a single phase, as is conventionally known in the art. Theterm “composite oxide” as used herein generally refers to a compositionof oxides having more than one phase, as is conventionally known in theart.

The expression “substantially free of” as used herein with reference toa material means that the material may be present in a minor amount,such as ≤5% by weight, preferably ≤2% by weight, more preferably ≤1% byweight. The expression “substantially free of” embraces the expression“does not comprise”. The term “loading” as used herein refers to ameasurement in units of g/ft³ on a metal weight basis.

DETAILED DESCRIPTION OF THE INVENTION

The lean NO_(x) trap catalyst of the invention comprises:

-   -   i) a first layer, said first layer comprising one or more        platinum group metals, a first ceria-containing material, an        alkali or alkaline earth metal, and a first inorganic oxide;    -   ii) a second layer, said second layer comprising one or more        noble metals, a second inorganic oxide, and a second        ceria-containing material; and    -   iii) a third layer, said third layer comprising one or more        noble metals having reducing activity and a third inorganic        oxide.

The one or more platinum group metals is preferably selected from thegroup consisting of palladium, platinum, rhodium, and mixtures thereof.Particularly preferably, the one or more platinum group metals is amixture or alloy of platinum and palladium, preferably wherein the ratioof platinum to palladium is from 2:1 to 12:1 on a w/w basis, especiallypreferably about 5:1 on a w/w basis.

The lean NO_(x) trap catalyst preferably comprises 0.1 to 10 weightpercent PGM, more preferably 0.5 to 5 weight percent PGM, and mostpreferably 1 to 3 weight percent PGM. The PGM is preferably present inan amount of 1 to 100 g/ft³, more preferably 10 to 80 g/ft³, mostpreferably 20 to 60 g/ft³.

Preferably the one or more platinum group metals do not comprise orconsist of rhodium. In other words, the first layer is preferablysubstantially free of rhodium.

The one or more platinum group metals are generally in contact with thefirst ceria-containing material. Preferably the one or more platinumgroup metals are supported on the first ceria-containing material.Alternatively or additionally, the one or more platinum group metals aresupported on the first inorganic oxide.

The first ceria-containing material is preferably selected from thegroup consisting of cerium oxide, a ceria-zirconia mixed oxide, and analumina-ceria-zirconia mixed oxide. Preferably the firstceria-containing material comprises bulk ceria. The firstceria-containing material may function as an oxygen storage material.Alternatively, or in addition, the first ceria-containing material mayfunction as a NO_(x) storage material, and/or as a support material forthe one or more platinum group metals and/or the alkali or alkalineearth metal.

The alkali or alkaline earth metal may be deposited on the firstceria-containing material. Alternatively, or in addition, the alkali oralkaline earth metal may be deposited on the first inorganic oxide. Thatis, in some embodiments, the alkali or alkaline earth metal may bedeposited on, i.e. present on, both the first ceria-containing materialand the first inorganic oxide.

The alkali or alkaline earth metal is generally in contact with thefirst inorganic oxide. Preferably the alkali or alkaline earth metal issupported on the first inorganic oxide. In addition to, or alternativelyto, being in contact with the first inorganic oxide, the alkali oralkaline earth metal may be in contact with the first ceria-containingmaterial.

The alkali or alkaline earth metal is preferably barium. Barium, wherepresent, is included as a NO_(x) storage material, i.e. the first layermay be a NO_(x) storage layer. Preferably the barium, where present, ispresent in an amount of 0.1 to 10 wt %, and more preferably 0.5 to 5weight percent barium, e.g. about 4.5 weight percent barium, expressedas a weight % of the composition.

Preferably the barium is present as a CeO₂—BaCO₃ composite material.Such a material can be preformed by any method known in the art, forexample incipient wetness impregnation or spray-drying.

The first inorganic oxide is preferably an oxide of Groups 2, 3, 4, 5,13 and 14 elements The first inorganic oxide is preferably selected fromthe group consisting of alumina, ceria, magnesia, silica, titania,zirconia, niobia, tantalum oxides, molybdenum oxides, tungsten oxides,and mixed oxides or composite oxides thereof. Particularly preferably,the first inorganic oxide is alumina, ceria, or a magnesia/aluminacomposite oxide. One especially preferred inorganic oxide is amagnesia/alumina composite oxide.

The first inorganic oxide may be a support material for the one or moreplatinum group metals, and/or for the alkali or alkaline earth metal.

Preferred first inorganic oxides preferably have a surface area in therange 10 to 1500 m²/g, pore volumes in the range 0.1 to 4 mL/g, and porediameters from about 10 to 1000 Angstroms. High surface area inorganicoxides having a surface area greater than 80 m²/g are particularlypreferred, e.g. high surface area ceria or alumina. Other preferredfirst inorganic oxides include magnesia/alumina composite oxides,optionally further comprising a cerium-containing component, e.g. ceria.In such cases the ceria may be present on the surface of themagnesia/alumina composite oxide, e.g. as a coating.

The one or more noble metals is preferably selected from the groupconsisting of palladium, platinum, rhodium, silver, gold, and mixturesthereof. Particularly preferably, the one or more noble metals is amixture or alloy of platinum and palladium, preferably wherein the ratioof platinum to palladium is from 2:1 to 10:1 on a w/w basis, especiallypreferably about 5:1 on a w/w basis.

Preferably the one or more noble metals do not comprise or consist ofrhodium. In other words, the second layer is preferably substantiallyfree of rhodium. In some embodiments therefore the first layer and thesecond layer are preferably substantially free of rhodium. This may beadvantageous as rhodium can negatively affect the catalytic activity ofother catalytic metals, such as platinum, palladium, or mixtures and/oralloys thereof.

The one or more noble metals are generally in contact with the secondceria-containing material. Preferably the one or more noble metals aresupported on the second ceria-containing material. In addition to, oralternatively to, being in contact with the second ceria-containingmaterial, the one or more noble metals may be in contact with the secondinorganic oxide.

The second inorganic oxide is preferably an oxide of Groups 2, 3, 4, 5,13 and 14 elements The second inorganic oxide is preferably selectedfrom the group consisting of alumina, ceria, magnesia, silica, titania,zirconia, niobia, tantalum oxides, molybdenum oxides, tungsten oxides,and mixed oxides or composite oxides thereof. Particularly preferably,the second inorganic oxide is alumina, ceria, or a magnesia/aluminacomposite oxide. One especially preferred second inorganic oxide isalumina.

The second inorganic oxide may be a support material for the one or morenoble metals.

Preferred second inorganic oxides preferably have a surface area in therange 10 to 1500 m²/g, pore volumes in the range 0.1 to 4 mL/g, and porediameters from about 10 to 1000 Angstroms. High surface area inorganicoxides having a surface area greater than 80 m²/g are particularlypreferred, e.g. high surface area ceria or alumina. Other preferredsecond inorganic oxides include magnesia/alumina composite oxides,optionally further comprising a cerium-containing component, e.g. ceria.In such cases the ceria may be present on the surface of themagnesia/alumina composite oxide, e.g. as a coating.

The second ceria-containing material is preferably selected from thegroup consisting of cerium oxide, a ceria-zirconia mixed oxide, and analumina-ceria-zirconia mixed oxide. Preferably the secondceria-containing material comprises bulk ceria. The secondceria-containing material may function as an oxygen storage material.Alternatively, or in addition, the second ceria-containing material mayfunction as a NO_(x) storage material, and/or as a support material forthe one or more noble metals.

The second layer may function as an oxidation layer, e.g. a dieseloxidation catalyst layer suitable for the oxidation of hydrocarbons toCO₂ and/or CO, and/or suitable for the oxidation of NO to NO₂.

In some preferred lean NO_(x) trap catalysts of the invention, the totalloading of the one or more platinum group metals in the first layer islower than the total loading of the one or more noble metals in thesecond layer. In such catalysts, preferably the ratio of the totalloading of the one or more noble metals in the second layer to the totalloading of the one or more platinum group metals in the first layer isat least 2:1 on a w/w basis.

In further preferred lean NO_(x) trap catalysts of the invention, thetotal loading of the first ceria-containing material is greater than thetotal loading of the second ceria-containing material. In suchcatalysts, preferably the ratio of the total loading of the firstceria-containing material is greater than the total loading of thesecond ceria-containing material by at least 2:1 on a w/w basis,preferably at least 3:1 on a w/w basis, more preferably at least 5:1 ona w/w basis, particularly preferably at least 7:1 on a w/w basis.

It has surprisingly been found that lean NO_(x) trap catalysts in whichthe total loading of the one or more platinum group metals in the firstlayer is lower than the total loading of the one or more noble metals inthe second layer, and/or the total loading of the first ceria-containingmaterial is greater than the total loading of the secondceria-containing material, have improved catalytic performance. Suchcatalysts have been found to show greater NO_(x) storage properties andCO oxidation activity compared to lean NO_(x) trap catalysts of the art.

It has further surprisingly been found that lean NO_(x) trap catalystsas described herein in which a ceria-containing material, e.g. ceria, ispresent in the second layer, have improved performance relative to anequivalent catalyst that does not contain a ceria-containing material inthe second layer. This finding is particularly surprising in that it isexpected that the presence of a ceria-containing material, e.g. ceria,in the second layer would lead to a decrease in the oxidation of NO toNO₂, as ceria would be expected to catalyst the reverse reaction, i.e.reduce NO₂. The inventors have surprisingly found, however, thatcontrary to this expectation that lean NO_(x) trap catalysts asdescribed herein demonstrate this improved performance under both leanand rich conditions.

Without wishing to be bound by theory, it is thought that thearrangement described above, in which the relative loading of the one ormore platinum group metals in the first layer is lower than that of theone or more noble metals in the second layer, and/or in which therelative loading of the first ceria-containing material (i.e. in thefirst layer) is higher than that of the second ceria-containing material(i.e. in the second layer), produces a separation of the NO_(x) storageand oxidation functions of the lean NO_(x) trap catalyst into separatelayers. In doing so, there is a synergistic benefit in which theseparated functions each individually have increased performancerelative to an equivalent catalyst in which oxidation and NO_(x) storagefunctions are located within the same layer.

The one or more noble metals having reducing activity is preferablyselected from the group consisting of palladium, platinum, rhodium,silver, gold, and mixtures thereof. Particularly preferably, the one ormore noble metals having reducing activity comprises, consistsessentially of, or consists of rhodium. One particularly preferred noblemetal having reducing activity is a mixture or alloy of rhodium andplatinum, preferably wherein the ratio of rhodium to platinum is from1:2 to 2:1 on a w/w basis, and especially preferably wherein the ratioof rhodium to platinum is about 1:1 on a w/w basis. In other words, oneparticularly preferred noble metal having reducing activity is a 1:1mixture or alloy of rhodium and platinum.

The one or more noble metals having reducing activity are generally incontact with the third inorganic oxide. The one or more noble metalshaving reducing activity are preferably supported on the third inorganicoxide.

The third inorganic oxide is preferably an oxide of Groups 2, 3, 4, 5,13 and 14 elements The third inorganic oxide is selected from the groupconsisting of alumina, ceria, magnesia, silica, titania, zirconia,niobia, tantalum oxides, molybdenum oxides, tungsten oxides, and mixedoxides or composite oxides thereof. Particularly preferably, the thirdinorganic oxide is alumina, ceria, or a magnesia/alumina compositeoxide. One especially preferred third inorganic oxide is ceria.

Preferred third inorganic oxides preferably have a surface area in therange 10 to 1500 m²/g, pore volumes in the range 0.1 to 4 mL/g, and porediameters from about 10 to 1000 Angstroms. High surface area inorganicoxides having a surface area greater than 80 m²/g are particularlypreferred, e.g. high surface area ceria or alumina. Other preferredthird inorganic oxides include magnesia/alumina composite oxides,optionally further comprising a cerium-containing component, e.g. ceria.In such cases the ceria may be present on the surface of themagnesia/alumina composite oxide, e.g. as a coating.

The lean NO_(x) trap catalysts of the invention may comprise furthercomponents that are known to the skilled person. For example, thecompositions of the invention may further comprise at least one binderand/or at least one surfactant. Where a binder is present, dispersiblealumina binders are preferred.

The lean NO_(x) trap catalysts of the invention may preferably furthercomprise a metal or ceramic substrate having an axial length L.Preferably the substrate is a flow-through monolith or a filtermonolith, but is preferably a flow-through monolith substrate.

The flow-through monolith substrate has a first face and a second facedefining a longitudinal direction therebetween. The flow-throughmonolith substrate has a plurality of channels extending between thefirst face and the second face. The plurality of channels extend in thelongitudinal direction and provide a plurality of inner surfaces (e.g.the surfaces of the walls defining each channel). Each of the pluralityof channels has an opening at the first face and an opening at thesecond face. For the avoidance of doubt, the flow-through monolithsubstrate is not a wall flow filter.

The first face is typically at an inlet end of the substrate and thesecond face is at an outlet end of the substrate.

The channels may be of a constant width and each plurality of channelsmay have a uniform channel width.

Preferably within a plane orthogonal to the longitudinal direction, themonolith substrate has from 100 to 500 channels per square inch,preferably from 200 to 400. For example, on the first face, the densityof open first channels and closed second channels is from 200 to 400channels per square inch. The channels can have cross sections that arerectangular, square, circular, oval, triangular, hexagonal, or otherpolygonal shapes.

The monolith substrate acts as a support for holding catalytic material.Suitable materials for forming the monolith substrate includeceramic-like materials such as cordierite, silicon carbide, siliconnitride, zirconia, mullite, spodumene, alumina-silica magnesia orzirconium silicate, or of porous, refractory metal. Such materials andtheir use in the manufacture of porous monolith substrates is well knownin the art.

It should be noted that the flow-through monolith substrate describedherein is a single component (i.e. a single brick). Nonetheless, whenforming an emission treatment system, the monolith used may be formed byadhering together a plurality of channels or by adhering together aplurality of smaller monoliths as described herein. Such techniques arewell known in the art, as well as suitable casings and configurations ofthe emission treatment system.

In embodiments wherein the lean NO_(x) trap catalyst comprises a ceramicsubstrate, the ceramic substrate may be made of any suitable refractorymaterial, e.g., alumina, silica, titania, ceria, zirconia, magnesia,zeolites, silicon nitride, silicon carbide, zirconium silicates,magnesium silicates, aluminosilicates and metallo aluminosilicates (suchas cordierite and spodumene), or a mixture or mixed oxide of any two ormore thereof. Cordierite, a magnesium aluminosilicate, and siliconcarbide are particularly preferred.

In embodiments wherein the lean NO_(x) trap catalyst comprises ametallic substrate, the metallic substrate may be made of any suitablemetal, and in particular heat-resistant metals and metal alloys such astitanium and stainless steel as well as ferritic alloys containing iron,nickel, chromium, and/or aluminium in addition to other trace metals.

The lean NO_(x) trap catalysts of the invention may be prepared by anysuitable means. For example, the first layer may be prepared by mixingthe one or more platinum group metals, a first ceria-containingmaterial, an alkali or alkaline earth metal, and a first inorganic oxidein any order. The manner and order of addition is not considered to beparticularly critical. For example, each of the components of the firstlayer may be added to any other component or components simultaneously,or may be added sequentially in any order. Each of the components of thefirst layer may be added to any other component of the first layer byimpregnation, adsorption, ion-exchange, incipient wetness,precipitation, or the like, or by any other means commonly known in theart.

The second layer may be prepared by mixing the one or more noble metals,a second ceria-containing material, and a second inorganic oxide in anyorder. The manner and order of addition is not considered to beparticularly critical. For example, each of the components of the secondlayer may be added to any other component or components simultaneously,or may be added sequentially in any order. Each of the components of thesecond layer may be added to any other component of the second layer byimpregnation, adsorption, ion-exchange, incipient wetness,precipitation, or the like, or by any other means commonly known in theart.

The third layer may be prepared by mixing the one or more noble metalshaving reducing activity and a third inorganic oxide. The manner andorder of addition is not considered to be particularly critical. Forexample, each of the components of the third layer may be added to anyother component or components simultaneously, or may be addedsequentially in any order. Each of the components of the third layer maybe added to any other component of the third layer by impregnation,adsorption, ion-exchange, incipient wetness, precipitation, or the like,or by any other means commonly known in the art.

Preferably, the lean NO_(x) trap catalyst as hereinbefore described isprepared by depositing the lean NO_(x) trap catalyst on the substrateusing washcoat procedures. A representative process for preparing thelean NO_(x) trap catalyst using a washcoat procedure is set forth below.It will be understood that the process below can be varied according todifferent embodiments of the invention.

The washcoating is preferably performed by first slurrying finelydivided particles of the components of the lean NO_(x) trap catalyst ashereinbefore defined in an appropriate solvent, preferably water, toform a slurry. The slurry preferably contains between 5 to 70 weightpercent solids, more preferably between 10 to 50 weight percent.Preferably, the particles are milled or subject to another comminutionprocess in order to ensure that substantially all of the solid particleshave a particle size of less than 20 microns in an average diameter,prior to forming the slurry. Additional components, such as stabilizers,binders, surfactants or promoters, may also be incorporated in theslurry as a mixture of water soluble or water-dispersible compounds orcomplexes.

The substrate may then be coated one or more times with the slurry suchthat there will be deposited on the substrate the desired loading of thelean NO_(x) trap catalyst.

Preferably the first layer is supported/deposited directly on the metalor ceramic substrate. By “directly on” it is meant that there are nointervening or underlying layers present between the first layer and themetal or ceramic substrate.

Preferably the second layer is deposited on the first layer.Particularly preferably the second layer is deposited directly on thefirst layer. By “directly on” it is meant that there are no interveningor underlying layers present between the second layer and the firstlayer.

Preferably the third layer is deposited on the second layer.Particularly preferably the third layer is deposited directly on thesecond layer. By “directly on” it is meant that there are no interveningor underlying layers present between the third layer and the secondlayer.

Thus in a preferred lean NO_(x) trap catalyst of the invention, thefirst layer is deposited directly on metal or ceramic substrate, thesecond layer is deposited on the first layer, and the third layer isdeposited on the second layer. Such lean NO_(x) trap catalysts may beconsidered to be a three layer lean NO_(x) trap.

Preferably the first layer, second layer and/or third layer aredeposited on at least 60% of the axial length L of the substrate, morepreferably on at least 70% of the axial length L of the substrate, andparticularly preferably on at least 80% of the axial length L of thesubstrate.

In particularly preferred lean NO_(x) trap catalysts of the invention,the first layer and the second layer are deposited on at least 80%,preferably at least 95%, of the axial length L of the substrate. In somepreferred lean NO_(x) trap catalysts of the invention, the third layeris deposited on less than 100% of the axial length L of the substrate,e.g. the third layer is deposited on 80-95% of the axial length L of thesubstrate, such as 80%, 85%, 90%, or 95% of the axial length L of thesubstrate. Thus in some particularly preferred lean NO_(x) trapcatalysts of the invention, the first layer and the second layer aredeposited on at least 95% of the axial length L of the substrate and thethird layer is deposited on 80-95% of the axial length L of thesubstrate, such as 80%, 85%, 90%, or 95% of the axial length L of thesubstrate.

Preferably, the lean NO_(x) trap catalyst comprises a substrate and atleast one layer on the substrate. Preferably, the at least one layercomprises the first layer as hereinbefore described. This can beproduced by the washcoat procedure described above. One or moreadditional layers may be added to the one layer of NO_(x) adsorbercatalyst composition, such as, but not limited to, the second layer ashereinbefore described and the third layer as hereinbefore described.

In embodiments wherein one or more additional layers are present inaddition to the first layer, the second layer and the third layer ashereinbefore described, the one or more additional layers have adifferent composition to the first layer, the second layer and the thirdlayer as hereinbefore described

The one or more additional layers may comprise one zone or a pluralityof zones, e.g. two or more zones. Where the one or more additionallayers comprise a plurality of zones, the zones are preferablylongitudinal zones. The plurality of zones, or each individual zone, mayalso be present as a gradient, i.e. a zone may not be of a uniformthickness along its entire length, to form a gradient. Alternatively azone may be of uniform thickness along its entire length.

In some preferred embodiments, one additional layer, i.e. a firstadditional layer, is present.

Typically, the first additional layer comprises a platinum group metal(PGM) (referred to below as the “second platinum group metal”). It isgenerally preferred that the first additional layer comprises the secondplatinum group metal (PGM) as the only platinum group metal (i.e. thereare no other PGM components present in the catalytic material, exceptfor those specified).

The second PGM may be selected from the group consisting of platinum,palladium, and a combination or mixture of platinum (Pt) and palladium(Pd). Preferably, the platinum group metal is selected from the groupconsisting of palladium (Pd) and a combination or a mixture of platinum(Pt) and palladium (Pd). More preferably, the platinum group metal isselected from the group consisting of a combination or a mixture ofplatinum (Pt) and palladium (Pd).

It is generally preferred that the first additional layer is (i.e. isformulated) for the oxidation of carbon monoxide (CO) and/orhydrocarbons (HCs).

Preferably, the first additional layer comprises palladium (Pd) andoptionally platinum (Pt) in a ratio by weight of 1:0 (e.g. Pd only) to1:4 (this is equivalent to a ratio by weight of Pt:Pd of 4:1 to 0:1).More preferably, the second layer comprises platinum (Pt) and palladium(Pd) in a ratio by weight of <4:1, such as ≤3.5:1.

When the platinum group metal is a combination or mixture of platinumand palladium, then the first additional layer comprises platinum (Pt)and palladium (Pd) in a ratio by weight of 5:1 to 3.5:1, preferably2.5:1 to 1:2.5, more preferably 1:1 to 2:1.

The first additional layer typically further comprises a supportmaterial (referred to herein below as the “second support material”).The second PGM is generally disposed or supported on the second supportmaterial.

The second support material is preferably a refractory oxide. It ispreferred that the refractory oxide is selected from the groupconsisting of alumina, silica, ceria, silica alumina, ceria-alumina,ceria-zirconia and alumina-magnesium oxide. More preferably, therefractory oxide is selected from the group consisting of alumina,ceria, silica-alumina and ceria-zirconia. Even more preferably, therefractory oxide is alumina or silica-alumina, particularlysilica-alumina.

A particularly preferred first additional layer comprises asilica-alumina support, platinum, palladium, barium, a molecular sieve,and a platinum group metal (PGM) on an alumina support, e.g. a rareearth-stabilised alumina. Particularly preferably, this preferred firstadditional layer comprises a first zone comprising a silica-aluminasupport, platinum, palladium, barium, a molecular sieve, and a secondzone comprising a platinum group metal (PGM) on an alumina support, e.g.a rare earth-stabilised alumina. This preferred first additional layermay have activity as an oxidation catalyst, e.g. as a diesel oxidationcatalyst (DOC).

A further preferred first additional layer layer comprises, consists of,or consists essentially of a platinum group metal on alumina. Thispreferred second layer may have activity as an oxidation catalyst, e.g.as a NO₂-maker catalyst.

A further preferred first additional layer comprises a platinum groupmetal, rhodium, and a cerium-containing component.

In other preferred embodiments, more than one of the preferred firstadditional layers described above are present, in addition to the leanNO_(x) trap catalyst. In such embodiments, the one or more additionallayers may be present in any configuration, including zonedconfigurations.

Preferably the first additional layer is disposed or supported on thelean NO_(x) trap catalyst.

The first additional layer may, additionally or alternatively, bedisposed or supported on the substrate (e.g. the plurality of innersurfaces of the through-flow monolith substrate).

The first additional layer may be disposed or supported on the entirelength of the substrate or the lean NO_(x) trap catalyst. Alternativelythe first additional layer may be disposed or supported on a portion,e.g. 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 95%, of thesubstrate or the lean NO_(x) trap catalyst.

Alternatively, the first layer, second layer and/or third layer may beextruded to form a flow-through or filter substrate. In such cases thelean NO_(x) trap catalyst is an extruded lean NO_(x) trap catalystcomprising the first layer, second layer and/or third layer ashereinbefore described.

A further aspect of the invention is an emission treatment system fortreating a flow of a combustion exhaust gas comprising the lean NO_(x)trap catalyst as hereinbefore defined. In preferred systems, theinternal combustion engine is a diesel engine, preferably a light dutydiesel engine. The lean NO_(x) trap catalyst may be placed in aclose-coupled position or in the underfloor position.

The emission treatment system typically further comprises an emissionscontrol device.

The emissions control devices is preferably downstream of the leanNO_(x) trap catalyst.

Examples of an emissions control device include a diesel particulatefilter (DPF), a lean NO_(x) trap (LNT), a lean NO_(x) catalyst (LNC), aselective catalytic reduction (SCR) catalyst, a diesel oxidationcatalyst (DOC), a catalysed soot filter (CSF), a selective catalyticreduction filter (SCRF™) catalyst, an ammonia slip catalyst (ASC), acold start catalyst (dCSC™) and combinations of two or more thereof.Such emissions control devices are all well known in the art.

Some of the aforementioned emissions control devices have filteringsubstrates. An emissions control device having a filtering substrate maybe selected from the group consisting of a diesel particulate filter(DPF), a catalysed soot filter (CSF), and a selective catalyticreduction filter (SCRF™) catalyst.

It is preferred that the emission treatment system comprises anemissions control device selected from the group consisting of a leanNO_(x) trap (LNT), an ammonia slip catalyst (ASC), diesel particulatefilter (DPF), a selective catalytic reduction (SCR) catalyst, acatalysed soot filter (CSF), a selective catalytic reduction filter(SCRF™) catalyst, and combinations of two or more thereof. Morepreferably, the emissions control device is selected from the groupconsisting of a diesel particulate filter (DPF), a selective catalyticreduction (SCR) catalyst, a catalysed soot filter (CSF), a selectivecatalytic reduction filter (SCRF™) catalyst, and combinations of two ormore thereof. Even more preferably, the emissions control device is aselective catalytic reduction (SCR) catalyst or a selective catalyticreduction filter (SCRF™) catalyst.

When the emission treatment system of the invention comprises an SCRcatalyst or an SCRF™ catalyst, then the emission treatment system mayfurther comprise an injector for injecting a nitrogenous reductant, suchas ammonia, or an ammonia precursor, such as urea or ammonium formate,preferably urea, into exhaust gas downstream of the lean NO_(x) trapcatalyst and upstream of the SCR catalyst or the SCRF™ catalyst.

Such an injector may be fluidly linked to a source (e.g. a tank) of anitrogenous reductant precursor. Valve-controlled dosing of theprecursor into the exhaust gas may be regulated by suitably programmedengine management means and closed loop or open loop feedback providedby sensors monitoring the composition of the exhaust gas.

Ammonia can also be generated by heating ammonium carbamate (a solid)and the ammonia generated can be injected into the exhaust gas.

Alternatively or in addition to the injector, ammonia can be generatedin situ (e.g. during rich regeneration of a LNT disposed upstream of theSCR catalyst or the SCRF™ catalyst, e.g. a lean NO_(x) trap catalyst ofthe invention). Thus, the emission treatment system may further comprisean engine management means for enriching the exhaust gas withhydrocarbons.

The SCR catalyst or the SCRF™ catalyst may comprise a metal selectedfrom the group consisting of at least one of Cu, Hf, La, Au, In, V,lanthanides and Group VIII transition metals (e.g. Fe), wherein themetal is supported on a refractory oxide or molecular sieve. The metalis preferably selected from Ce, Fe, Cu and combinations of any two ormore thereof, more preferably the metal is Fe or Cu.

The refractory oxide for the SCR catalyst or the SCRF™ catalyst may beselected from the group consisting of Al₂O₃, TiO₂, CeO₂, SiO₂, ZrO₂ andmixed oxides containing two or more thereof. The non-zeolite catalystcan also include tungsten oxide (e.g. V₂O₅/WO₃/TiO₂, WO_(x)/CeZrO₂,WO_(x)/ZrO₂ or Fe/WO_(x)/ZrO₂).

It is particularly preferred when an SCR catalyst, an SCRF™ catalyst ora washcoat thereof comprises at least one molecular sieve, such as analuminosilicate zeolite or a SAPO. The at least one molecular sieve canbe a small, a medium or a large pore molecular sieve. By “small poremolecular sieve” herein we mean molecular sieves containing a maximumring size of 8, such as CHA; by “medium pore molecular sieve” herein wemean a molecular sieve containing a maximum ring size of 10, such asZSM-5; and by “large pore molecular sieve” herein we mean a molecularsieve having a maximum ring size of 12, such as beta. Small poremolecular sieves are potentially advantageous for use in SCR catalysts.

In the emission treatment system of the invention, preferred molecularsieves for an SCR catalyst or an SCRF™ catalyst are syntheticaluminosilicate zeolite molecular sieves selected from the groupconsisting of AEI, ZSM-5, ZSM-20, ERI including ZSM-34, mordenite,ferrierite, BEA including Beta, Y, CHA, LEV including Nu-3, MCM-22 andEU-1, preferably AEI or CHA, and having a silica-to-alumina ratio ofabout 10 to about 50, such as about 15 to about 40.

In a first emission treatment system embodiment, the emission treatmentsystem comprises the lean NO_(x) trap catalyst of the invention and acatalysed soot filter (CSF). The lean NO_(x) trap catalyst is typicallyfollowed by (e.g. is upstream of) the catalysed soot filter (CSF). Thus,for example, an outlet of the lean NO_(x) trap catalyst is connected toan inlet of the catalysed soot filter.

A second emission treatment system embodiment relates to an emissiontreatment system comprising the lean NO_(x) trap catalyst of theinvention, a catalysed soot filter (CSF) and a selective catalyticreduction (SCR) catalyst.

The lean NO_(x) trap catalyst is typically followed by (e.g. is upstreamof) the catalysed soot filter (CSF). The catalysed soot filter istypically followed by (e.g. is upstream of) the selective catalyticreduction (SCR) catalyst. A nitrogenous reductant injector may bearranged between the catalysed soot filter (CSF) and the selectivecatalytic reduction (SCR) catalyst. Thus, the catalysed soot filter(CSF) may be followed by (e.g. is upstream of) a nitrogenous reductantinjector, and the nitrogenous reductant injector may be followed by(e.g. is upstream of) the selective catalytic reduction (SCR) catalyst.

In a third emission treatment system embodiment, the emission treatmentsystem comprises the lean NO_(x) trap catalyst of the invention, aselective catalytic reduction (SCR) catalyst and either a catalysed sootfilter (CSF) or a diesel particulate filter (DPF).

In the third emission treatment system embodiment, the lean NO_(x) trapcatalyst of the invention is typically followed by (e.g. is upstream of)the selective catalytic reduction (SCR) catalyst. A nitrogenousreductant injector may be arranged between the oxidation catalyst andthe selective catalytic reduction (SCR) catalyst. Thus, the catalyzedmonolith substrate may be followed by (e.g. is upstream of) anitrogenous reductant injector, and the nitrogenous reductant injectormay be followed by (e.g. is upstream of) the selective catalyticreduction (SCR) catalyst. The selective catalytic reduction (SCR)catalyst are followed by (e.g. are upstream of) the catalysed sootfilter (CSF) or the diesel particulate filter (DPF).

A fourth emission treatment system embodiment comprises the lean NO_(x)trap catalyst of the invention and a selective catalytic reductionfilter (SCRF™) catalyst. The lean NO_(x) trap catalyst of the inventionis typically followed by (e.g. is upstream of) the selective catalyticreduction filter (SCRF™) catalyst.

A nitrogenous reductant injector may be arranged between the lean NO_(x)trap catalyst and the selective catalytic reduction filter (SCRF™)catalyst. Thus, the lean NO_(x) trap catalyst may be followed by (e.g.is upstream of) a nitrogenous reductant injector, and the nitrogenousreductant injector may be followed by (e.g. is upstream of) theselective catalytic reduction filter (SCRF™) catalyst.

When the emission treatment system comprises a selective catalyticreduction (SCR) catalyst or a selective catalytic reduction filter(SCRF™) catalyst, such as in the second to fourth exhaust systemembodiments described hereinabove, an ASC can be disposed downstreamfrom the SCR catalyst or the SCRF™ catalyst (i.e. as a separate monolithsubstrate), or more preferably a zone on a downstream or trailing end ofthe monolith substrate comprising the SCR catalyst can be used as asupport for the ASC.

Another aspect of the invention relates to a vehicle. The vehiclecomprises an internal combustion engine, preferably a diesel engine. Theinternal combustion engine preferably the diesel engine, is coupled toan emission treatment system of the invention.

It is preferred that the diesel engine is configured or adapted to runon fuel, preferably diesel fuel, comprising ≤50 ppm of sulfur, morepreferably ≤15 ppm of sulfur, such as ≤10 ppm of sulfur, and even morepreferably ≤5 ppm of sulfur.

The vehicle may be a light-duty diesel vehicle (LDV), such as defined inUS or European legislation. A light-duty diesel vehicle typically has aweight of <2840 kg, more preferably a weight of <2610 kg. In the US, alight-duty diesel vehicle (LDV) refers to a diesel vehicle having agross weight of ≤8,500 pounds (US lbs). In Europe, the term light-dutydiesel vehicle (LDV) refers to (i) passenger vehicles comprising no morethan eight seats in addition to the driver's seat and having a maximummass not exceeding 5 tonnes, and (ii) vehicles for the carriage of goodshaving a maximum mass not exceeding 12 tonnes.

Alternatively, the vehicle may be a heavy-duty diesel vehicle (HDV),such as a diesel vehicle having a gross weight of >8,500 pounds (USlbs), as defined in US legislation.

A further aspect of the invention is a method of treating an exhaust gasfrom an internal combustion engine comprising contacting the exhaust gaswith the lean NO_(x) trap catalyst as hereinbefore described. Inpreferred methods, the exhaust gas is a rich gas mixture. In furtherpreferred methods, the exhaust gas cycles between a rich gas mixture anda lean gas mixture.

In some preferred methods of treating an exhaust gas from an internalcombustion engine, the exhaust gas is at a temperature of about 150 to300° C.

In further preferred methods of treating an exhaust gas from an internalcombustion engine, the exhaust gas is contacted with one or more furtheremissions control devices, in addition to the lean NO_(x) trap catalystas hereinbefore described. The emissions control device or devices ispreferably downstream of the lean NO_(x) trap catalyst.

Examples of a further emissions control device include a dieselparticulate filter (DPF), a lean NO_(x) trap (LNT), a lean NO_(x)catalyst (LNC), a selective catalytic reduction (SCR) catalyst, a dieseloxidation catalyst (DOC), a catalysed soot filter (CSF), a selectivecatalytic reduction filter (SCRF™) catalyst, an ammonia slip catalyst(ASC), a cold start catalyst (dCSC™) and combinations of two or morethereof. Such emissions control devices are all well known in the art.

Some of the aforementioned emissions control devices have filteringsubstrates. An emissions control device having a filtering substrate maybe selected from the group consisting of a diesel particulate filter(DPF), a catalysed soot filter (CSF), and a selective catalyticreduction filter (SCRF™) catalyst.

It is preferred that the method comprises contacting the exhaust gaswith an emissions control device selected from the group consisting of alean NO_(x) trap (LNT), an ammonia slip catalyst (ASC), dieselparticulate filter (DPF), a selective catalytic reduction (SCR)catalyst, a catalysed soot filter (CSF), a selective catalytic reductionfilter (SCRF™) catalyst, and combinations of two or more thereof. Morepreferably, the emissions control device is selected from the groupconsisting of a diesel particulate filter (DPF), a selective catalyticreduction (SCR) catalyst, a catalysed soot filter (CSF), a selectivecatalytic reduction filter (SCRF™) catalyst, and combinations of two ormore thereof. Even more preferably, the emissions control device is aselective catalytic reduction (SCR) catalyst or a selective catalyticreduction filter (SCRF™) catalyst.

When the the method of the invention comprises contacting the exhaustgas with an SCR catalyst or an SCRF™ catalyst, then the method mayfurther comprise the injection of a nitrogenous reductant, such asammonia, or an ammonia precursor, such as urea or ammonium formate,preferably urea, into exhaust gas downstream of the lean NO_(x) trapcatalyst and upstream of the SCR catalyst or the SCRF™ catalyst.

Such an injection may be carried out by an injector. The injector may befluidly linked to a source (e.g. a tank) of a nitrogenous reductantprecursor. Valve-controlled dosing of the precursor into the exhaust gasmay be regulated by suitably programmed engine management means andclosed loop or open loop feedback provided by sensors monitoring thecomposition of the exhaust gas.

Ammonia can also be generated by heating ammonium carbamate (a solid)and the ammonia generated can be injected into the exhaust gas.

Alternatively or in addition to the injector, ammonia can be generatedin situ (e.g. during rich regeneration of a LNT disposed upstream of theSCR catalyst or the SCRF™ catalyst). Thus, the method may furthercomprise enriching of the exhaust gas with hydrocarbons.

The SCR catalyst or the SCRF™ catalyst may comprise a metal selectedfrom the group consisting of at least one of Cu, Hf, La, Au, In, V,lanthanides and Group VIII transition metals (e.g. Fe), wherein themetal is supported on a refractory oxide or molecular sieve. The metalis preferably selected from Ce, Fe, Cu and combinations of any two ormore thereof, more preferably the metal is Fe or Cu.

The refractory oxide for the SCR catalyst or the SCRF™ catalyst may beselected from the group consisting of Al₂O₃, TiO₂, CeO₂, SiO₂, ZrO₂ andmixed oxides containing two or more thereof. The non-zeolite catalystcan also include tungsten oxide (e.g. V₂O₅/WO₃/TiO₂, WO_(x)/CeZrO₂,WO_(x)/ZrO₂ or Fe/WO_(x)/ZrO₂).

It is particularly preferred when an SCR catalyst, an SCRF™ catalyst ora washcoat thereof comprises at least one molecular sieve, such as analuminosilicate zeolite or a SAPO. The at least one molecular sieve canbe a small, a medium or a large pore molecular sieve. By “small poremolecular sieve” herein we mean molecular sieves containing a maximumring size of 8, such as CHA; by “medium pore molecular sieve” herein wemean a molecular sieve containing a maximum ring size of 10, such asZSM-5; and by “large pore molecular sieve” herein we mean a molecularsieve having a maximum ring size of 12, such as beta. Small poremolecular sieves are potentially advantageous for use in SCR catalysts.

In the method of treating an exhaust gas of the invention, preferredmolecular sieves for an SCR catalyst or an SCRF™ catalyst are syntheticaluminosilicate zeolite molecular sieves selected from the groupconsisting of AEI, ZSM-5, ZSM-20, ERI including ZSM-34, mordenite,ferrierite, BEA including Beta, Y, CHA, LEV including Nu-3, MCM-22 andEU-1, preferably AEI or CHA, and having a silica-to-alumina ratio ofabout 10 to about 50, such as about 15 to about 40.

In a first embodiment, the method comprises contacting the exhaust gaswith the lean NO_(x) trap catalyst of the invention and a catalysed sootfilter (CSF). The lean NO_(x) trap catalyst is typically followed by(e.g. is upstream of) the catalysed soot filter (CSF). Thus, forexample, an outlet of the lean NO_(x) trap catalyst is connected to aninlet of the catalysed soot filter.

A second embodiment of the method of treating an exhaust gas relates toa method comprising contacting the exhaust gas with the lean NO_(x) trapcatalyst of the invention, a catalysed soot filter (CSF) and a selectivecatalytic reduction (SCR) catalyst.

The lean NO_(x) trap catalyst is typically followed by (e.g. is upstreamof) the catalysed soot filter (CSF). The catalysed soot filter istypically followed by (e.g. is upstream of) the selective catalyticreduction (SCR) catalyst. A nitrogenous reductant injector may bearranged between the catalysed soot filter (CSF) and the selectivecatalytic reduction (SCR) catalyst. Thus, the catalysed soot filter(CSF) may be followed by (e.g. is upstream of) a nitrogenous reductantinjector, and the nitrogenous reductant injector may be followed by(e.g. is upstream of) the selective catalytic reduction (SCR) catalyst.

In a third embodiment of the method of treating an exhaust gas, themethod comprises contacting the exhaust gas with the lean NO_(x) trapcatalyst of the invention, a selective catalytic reduction (SCR)catalyst and either a catalysed soot filter (CSF) or a dieselparticulate filter (DPF).

In the third embodiment of the method of treating an exhaust gas, thelean NO_(x) trap catalyst of the invention is typically followed by(e.g. is upstream of) the selective catalytic reduction (SCR) catalyst.A nitrogenous reductant injector may be arranged between the oxidationcatalyst and the selective catalytic reduction (SCR) catalyst. Thus, thelean NO_(x) trap catalyst may be followed by (e.g. is upstream of) anitrogenous reductant injector, and the nitrogenous reductant injectormay be followed by (e.g. is upstream of) the selective catalyticreduction (SCR) catalyst. The selective catalytic reduction (SCR)catalyst are followed by (e.g. are upstream of) the catalysed sootfilter (CSF) or the diesel particulate filter (DPF).

A fourth embodiment of the method of treating an exhaust gas comprisesthe lean NO_(x) trap catalyst of the invention and a selective catalyticreduction filter (SCRF™) catalyst. The lean NO_(x) trap catalyst of theinvention is typically followed by (e.g. is upstream of) the selectivecatalytic reduction filter (SCRF™) catalyst.

A nitrogenous reductant injector may be arranged between the lean NO_(x)trap catalyst and the selective catalytic reduction filter (SCRF™)catalyst. Thus, the lean NO_(x) trap catalyst may be followed by (e.g.is upstream of) a nitrogenous reductant injector, and the nitrogenousreductant injector may be followed by (e.g. is upstream of) theselective catalytic reduction filter (SCRF™) catalyst.

When the emission treatment system comprises a selective catalyticreduction (SCR) catalyst or a selective catalytic reduction filter(SCRF™) catalyst, such as in the second to fourth method embodimentsdescribed hereinabove, an ASC can be disposed downstream from the SCRcatalyst or the SCRF™ catalyst (i.e. as a separate monolith substrate),or more preferably a zone on a downstream or trailing end of themonolith substrate comprising the SCR catalyst can be used as a supportfor the ASC.

Examples

The invention will now be illustrated by the following non-limitingexamples.

Materials

All materials are commercially available and were obtained from knownsuppliers, unless noted otherwise.

General Preparation

Al₂O₃.CeO₂.MgO—BaCO₃ composite material was formed by impregnating Al₂O₃(56.14%).CeO₂ (6.52%).MgO (14.04%) with barium acetate and spray-dryingthe resultant slurry. This was followed by calcination at 650° C. for 1hour. Target BaCO₃ concentration is 23.3 wt %.

Example Preparation

Preparation of [Al₂O₃.CeO₂.MgO.Ba].Pt.Pd.CeO₂—Composition A

2.07 g/in³ [Al₂O₃.CeO₂.MgO.BaCO₃] (prepared according to the generalpreparation above) was made into a slurry with distilled water and thenmilled to reduce the average particle size (d₉₀=13-15 μm). To theslurry, 30 g/ft³ Pt malonate and 6 g/ft³ Pd nitrate solution were added,and stirred until homogenous. The Pt/Pd was allowed to adsorb onto thesupport for 1 hour. To this slurry was added 2.1 g/in³ of pre-calcinedCeO₂ followed by 0.2 g/in³ alumina binder, and stirred until homogenousto form a washcoat.

Preparation of [Al₂O₃.LaO].Pt.Pd.CeO₂—Composition B

Pt malonate (65 gft⁻³) and Pd nitrate (13 gft⁻³) were added to a slurryof [Al₂O₃ (90.0%).LaO (4%)](1.2 gin⁻³) in water. The Pt and Pd wereallowed to adsorb to the alumina support for 1 hour before CeO₂ (0.3gin⁻³) was added. The resultant slurry was made into a washcoat andthickened with natural thickener (hydroxyethylcellulose).

Preparation of [Al₂O₃.LaO].Pt.Pd—Composition C

Pt malonate (65 gft⁻³) and Pd nitrate (13 gft⁻³) were added to a slurryof [Al₂O₃ (90.0%).LaO (4%)](1.2 gin⁻³) in water. The Pt and Pd wereallowed to adsorb to the alumina support for 1 hour. The resultantslurry was made into a washcoat and thickened with natural thickener(hydroxyethylcellulose).

Preparation of [CeO₂].Rh.Pt.Al₂O₃—Composition D

Rh nitrate (5 gft⁻³) was added to a slurry of CeO₂ (0.4 gin⁻³) in water.Aqueous NH₃ was added until pH 6.8 to promote Rh adsorption. Followingthis, Pt malonate (5 gft⁻³) was added to the slurry and allowed toadsorb to the support for 1 hour before alumina (boehmite, 0.2 gin⁻³)and binder (alumina, 0.1 gin⁻³) were added. The resultant slurry wasmade into a washcoat.

Catalyst 1

Each of washcoats A, C and D were coated sequentially onto a ceramic ormetallic monolith using standard coating procedures, dried at 100° C.and calcined at 500° C. for 45 mins.

Catalyst 2

Each of washcoats A, B and D were coated sequentially onto a ceramic ormetallic monolith using standard coating procedures, dried at 100° C.and calcined at 500° C. for 45 mins.

Experimental Results

Catalysts 1 and 2 were hydrothermally aged at 800° C. for 16 h, in a gasstream consisting of 10% H₂O, 20% 02, and balance N₂. They wereperformance tested over a steady-state emissions cycle (three cycles of300 s lean and 10 s rich, with a target NO_(x) exposure of 1 g) using a1.6 litre bench mounted diesel engine. Emissions were measured pre- andpost-catalyst.

Example 1

The NO_(x) storage performance of the catalysts was assessed bymeasuring NO_(x) storage efficiency as a function of NO_(x) stored. Theresults from one representative cycle at 150° C., following adeactivating precondition, are shown in Table 1 below.

TABLE 1 NO_(x) stored NO_(x) storage efficiency (%) (g) Catalyst 1Catalyst 2 0.1 92 96 0.2 87 92 0.3 79 84 0.4 67 73 0.5 53 58 0.6 39 43

It can be seen from the results in Table 1 that Catalyst 2, comprising aCe-containing middle layer, has higher NO_(x) storage efficiency thanCatalyst 1, which does not comprise a Ce-containing middle layer.

Example 2

The NO_(x) storage performance of the catalysts was assessed bymeasuring NO_(x) storage efficiency as a function of NO_(x) stored. Theresults from one representative cycle at 150° C., following a moreactivating precondition than that of Example 1 above, are shown in Table2 below.

TABLE 2 NO_(x) stored NO_(x) storage efficiency (%) (g) Catalyst 1Catalyst 2 0.1 33 57 0.2 18 34 0.3 — 18 0.4 — — 0.5 — — 0.6 — —

It can be seen from the results in Table 2 that, similarly to in Example1 above, Catalyst 2, comprising a Ce-containing middle layer, has higherNO_(x) storage efficiency than Catalyst 1, which does not comprise aCe-containing middle layer.

Example 3

The NO_(x) storage performance of the catalysts was assessed bymeasuring NO_(x) storage efficiency as a function of NO_(x) stored. Theresults from one representative cycle at 200° C., following adeactivating precondition, are shown in Table 1 below.

TABLE 3 NO_(x) stored NOx storage efficiency (%) (g) Catalyst 1 Catalyst2 0.1 94 95 0.2 89 91 0.3 85 89 0.4 81 86 0.5 77 83 0.6 73 80

It can be seen from the results in Table 3 that Catalyst 2, comprising aCe-containing middle layer, has higher NO_(x) storage efficiency thanCatalyst 1, which does not comprise a Ce-containing middle layer.

Example 4

The NO_(x) storage performance of the catalysts was assessed bymeasuring NO_(x) storage efficiency as a function of NO_(x) stored. Theresults from one representative cycle at 200° C., following adeactivating precondition, are shown in Table 1 below.

TABLE 4 NO_(x) stored NOx storage efficiency (%) (g) Catalyst 1 Catalyst2 0.1 72 85 0.2 61 81 0.3 45 69 0.4 36 58 0.5 30 47 0.6 — 41

It can be seen from the results in Table 4 that Catalyst 2, comprising aCe-containing middle layer, has higher NO_(x) storage efficiency thanCatalyst 1, which does not comprise a Ce-containing middle layer.

Example 5

The CO oxidation performance of the catalysts was assessed by measuringCO conversion over time. The results from one representative cycle at175° C., following an activating steady state test condition, are shownin Table 5 below.

TABLE 5 CO conversion efficiency (%) Time (s) Catalyst 1 Catalyst 2 7512 17 100 20 36 125 70 90 150 96 98 175 99 99

It can be seen from the results in Table 5 that Catalyst 2, comprising aCe-containing middle layer, has higher CO conversion efficiency thanCatalyst 1, which does not comprise a Ce-containing middle layer.

This is further demonstrated by the time taken to each 25% and 50% COconversion efficiency at 175° C. for each catalyst. Catalyst 1 achieved25% CO conversion efficiency after 108 s, and 50% CO conversionefficiency after 121 s. Catalyst 2 achieved 25% CO conversion efficiencyafter 85 s, and 50% CO conversion efficiency after 110 s. Catalyst 2therefore achieves CO light-off sooner than Catalyst 1.

Example 6

The CO oxidation performance of the catalysts was assessed by measuringCO conversion over time. The results from one representative cycle at200° C., following an activating steady state test condition, are shownin Table 6 below.

TABLE 6 CO conversion efficiency (%) Time (s) Catalyst 1 Catalyst 2 7515 25 100 26 51 125 78 95 150 97 99 175 99 99

It can be seen from the results in Table 4 that Catalyst 2, comprising aCe-containing middle layer, has higher CO conversion efficiency thanCatalyst 1, which does not comprise a Ce-containing middle layer.

This is further demonstrated by the time taken to each 25% and 50% COconversion efficiency at 200° C. for each catalyst. Catalyst 1 achieved25% CO conversion efficiency after 97 s, and 50% CO conversionefficiency after 118 s. Catalyst 2 achieved 25% CO conversion efficiencyafter 76 s, and 50% CO conversion efficiency after 99 s. Catalyst 2therefore achieves CO light-off sooner than Catalyst 1.

The invention claimed is:
 1. A lean NO_(x) trap catalyst, comprising: i)a first layer, said first layer comprising one or more platinum groupmetals, a first ceria-containing material, an alkali or alkaline earthmetal, and a first inorganic oxide; ii) a second layer, said secondlayer comprising one or more noble metals, a second ceria-containingmaterial, and a second inorganic oxide; and iii) a third layer, saidthird layer comprising one or more noble metals having reducing activityand a third inorganic oxide; wherein the total loading of the one ormore platinum group metals in the first layer is lower than the totalloading of the one or more noble metals in the second layer.
 2. The leanNO_(x) trap catalyst of claim 1, wherein the ratio of the total loadingof the one or more noble metals in the second layer to the total loadingof the one or more platinum group metals in the first layer is at least2:1 on a w/w basis.
 3. The lean NO_(x) trap catalyst of claim 1, whereinthe total loading of the first ceria-containing material is greater thanthe total loading of the second ceria-containing material.
 4. The leanNO_(x) trap catalyst of claim 1, wherein the ratio of the total loadingof the first ceria-containing material is greater than the total loadingof the second ceria-containing material is at least 2:1 on a w/w basis.5. The lean NO_(x) trap catalyst of claim 1, wherein said one or moreplatinum group metals is a mixture or alloy of platinum and palladiumwherein the ratio of platinum to palladium is from 2:1 to 12:1 on a w/wbasis.
 6. The lean NO_(x) trap catalyst of claim 1, wherein the one ormore platinum group metals are supported on the first ceria-containingmaterial and the one or more noble metals are supported on the secondceria-containing material.
 7. The lean NO_(x) trap catalyst of claim 1,wherein said first ceria-containing material and the secondceria-containing material are independently selected from the groupconsisting of cerium oxide, a ceria-zirconia mixed oxide, and analumina-ceria-zirconia mixed oxide.
 8. The lean NO_(x) trap catalyst ofclaim 1, wherein the alkali or alkaline earth metal is barium whereinthe barium is present in an amount of 0.1 to 10 wt %.
 9. The lean NO_(x)trap catalyst of claim 1, wherein the first inorganic oxide, the secondinorganic oxide, and the third inorganic oxide are independentlyselected from the group consisting of alumina, ceria, magnesia, silica,titania, zirconia, niobia, tantalum oxides, molybdenum oxides, tungstenoxides, and mixed oxides or composite oxides thereof.
 10. The leanNO_(x) trap catalyst of claim 1, wherein the first inorganic oxide, thesecond inorganic oxide and the third inorganic oxide are independentlyalumina, ceria, or a magnesia/alumina composite oxide.
 11. The leanNO_(x) trap catalyst of claim 1, wherein the first inorganic oxide is amagnesia/alumina composite oxide, the second inorganic oxide is alumina,and the third inorganic oxide is ceria.
 12. The lean NO_(x) trapcatalyst of claim 1, wherein the one or more noble metals is selectedfrom the group consisting of palladium, platinum, rhodium, silver, gold,and mixtures thereof.
 13. The lean NO_(x) trap catalyst of claim 1,wherein the one or more noble metals is a mixture or alloy of platinumand palladium, wherein the platinum to palladium is from 2:1 to 10:1 ona w/w basis.
 14. The lean NO_(x) trap catalyst of claim 1, wherein theone or more noble metals having reducing activity is selected from thegroup consisting of palladium, platinum, rhodium, silver, gold, andmixtures thereof.
 15. The lean NO_(x) trap catalyst of claim 1, whereinthe one or more noble metals having reducing activity is a mixture oralloy of rhodium and platinum.
 16. The lean NO_(x) trap catalyst ofclaim 15, wherein the ratio of rhodium to platinum is from 1:2 to 2:1 ona w/w basis.
 17. The lean NO_(x) trap catalyst of claim 1, wherein theone or more noble metals having reducing activity are supported on thethird inorganic oxide.
 18. The lean NO_(x) trap catalyst of claim 1,further comprising a metal or ceramic substrate having an axial lengthL, wherein the substrate is a flow-through monolith or a filter monolithand wherein the first layer is deposited directly on the metal orceramic substrate, the second layer is deposited on the first layer, andthe third layer is deposited on the second layer.
 19. The lean NO_(x)trap catalyst of claim 1, wherein the first layer, second layer and/orthird layer are deposited on at least 60% of the axial length L of thesubstrate.